An intrusion detector is a device which generates an alarm signal when the detector detects unauthorized activity in or around a protected volume of space. The detector may be, for example, a motion sensor or a glass-break sensor.
After an installer installs the detector, the installer tests the detector to ensure that it is functioning properly. To test a motion sensor, for example, the installer walks throughout the motion sensor's field of view to cause the generation of a sensing signal. In response to the sensing signal, a detected-event indicator such as an LED may be illuminated.
To perform the test, the installer may be required to switch the detector into a test mode. This is conventionally done by opening the housing of the detector and installing a jumper or actuating a switch. However, many detectors are mounted in hard- to-reach locations, such as on ceilings. Switching such a hard-to-reach detector into a test mode is inconvenient, or even dangerous, for the installer.
Therefore, it is desirable to be able to switch the detector into a test mode remotely, obviating the need to open, or even physically contact, the detector. One prior art attempt at providing remote mode switching is the GlassTrek 450/450-S manufactured by Pirotec Technologies of St-Eustache, Quebec, Canada. An installer switches the GlassTrek 450/450S into a test mode by moving a switch on a Testtrek 1 glass break simulator to a Start/End position, pointing the Testtrek 1 at the GlassTrek 450/450-S, and pressing and holding a push switch on the Testtrek 1 for about three seconds. The GlassTrek 450/450-S switches into a test mode at the conclusion of the three seconds.
It appears that the TestTrek 1 remote unit emits a series of three continuous tones during the three second period over which the push switch is pressed and held and that the GlassTrek 450/450-S decodes the frequency, amplitude, and/or duration of the three continuous tones.
The prior art attempt has several drawbacks. First, substantial time is required for generation of the three continuous tones. During this time, movement of the Testtrek 1 remote unit obscures the frequency, amplitude and/or duration of the emitted tones.
Another problem in designing an acoustic signaling system for use in rooms is distortion of the received signal by reflections from the walls, floor and ceiling of the room. Because of such reflections, the prior art methods of modulating a signal onto a carrier waveform do not work well. The signal received by the microphone consists of the direct sound summed with time-delayed replicas along reflected paths. If the original emitted signal is a sound carrier amplitude- or frequency-modulated by a code waveform then the replicas interfere with it constructively or destructively. This effect is known as "multipath distortion" in radio-frequency signalling. If the delays in the reflected path signals are small, multipath distortion causes unpredictable amplitude variations in the received signal. If the delays are large compared with the duration of the code, not only amplitude variations but echoes will occur. As with echoes perceived by ear, the original signal may be made unintelligible to the receiver by the superposition of multiple overlapping replicas.
Multipath distortion is much more of a problem in acoustic signalling than it is in signalling methods based on electromagnetic waves, for two reasons. First, the speed of sound in air is much less than the speed of electromagnetic waves. Consequently, reflective paths in an ordinary room may introduce delays in the tens of milliseconds. Secondly, surfaces in ordinary rooms usually are reflective enough to allow significant secondary reflections of sound waves. This characteristic leads to reverberation, which is a condition of sustained reflections randomly distributed throughout the room.
Because of the distortion imposed on acoustic signals by multipath distortion in rooms, the prior art techniques of signaling do not work well in this situation. The prior art techniques involve modulating a carrier signal with a lower frequency waveform containing the intelligence. In the common techniques, such as continuous or discrete amplitude or frequency modulation, the carrier is "on" for a substantial portion of the transmission, varying typically from 50% to 100%. In acoustic signaling, the problem is that while the carrier is on, sound energy is radiated into the room, where it contributes to the system of reflections and reverberation, which then cause distortion at the receiver.